Characterizing Catalysts Via Temperature-Programmed Techniques

The Micromeritics Flow Reactor Series is an advanced modular laboratory system designed for measuring the activity and selectivity of catalysts. It is possible to re-configure and link these reactors to a mass spectrometer for pulse chemisorption and temperature-programmed methods used for the characterization and testing of catalysts in situ. 

Known as the Micro Catalyst Characterization and Testing Center (MCCTC) this new research tool enables manifold catalyst characterization and activity testing at pressures varying from atmospheric up to 100 bar or 200 bar, depending upon instrument configuration. 

Before testing the performance of the catalyst, characterization methods such as temperature-programmed oxidation (TPO), temperature-programmed reduction (TPR), and temperature-programmed desorption (TPD) can be employed as a point of reference. The capability to track transient activity on-line provides key data regarding the active life and deactivation behavior of the catalyst. 

Important and precise data regarding catalytic behavior can be obtained through the combination of characterization and performance testing in a single instrument. 

The Instrument

The MCCTC configuration of the Micromeritics Flow Reactor is shown in Figure 1. The Flow Reactor is designed for characterization and testing of a catalyst to determine its activity and selectivity. The key components of this apparatus include 3 six-port valves, mass flow controllers (MFCs), a heated reactor zone, and a quadrupole mass spectrometer for on-line assessment of gas mixtures. 

Valves and a liquid evaporator are accommodated in a hot box which is heated up to 200°C. This ensures proper mixing of gases and prevents condensation inside the system. Heated vapours are cooled and isolated from the gas phase by means of a patented liquid/gas separator. 

Figure 1. Flow Diagram of the MCCTC connected to a mass spectrometer.

Experiment 1: Temperature-Programmed Reduction (TPR)

TPR is used to characterize the reducible species present in the catalyst and to determine the impact of the catalyst support on active metal dispersion. In this experiment, the effect of H₂ pressure on a catalyst reduction profile was examined by obtaining TPR profiles at two different pressures: 1 bar and 25 bar. A catalyst mixture of silver (I) oxide and copper (II) oxide in an approximately 50:50 ratio was used in this experiment. A flow of 100 sccm nitrogen was supplied as a diluting agent and carrier gas, while a mixed flow of 50 sccm 10% hydrogen balance nitrogen was utilized for species reduction. 

TPR was then carried out on the copper and silver oxide mixture at atmospheric pressure (Figure 2). The same TPR analysis was then carried out at 25 bar pressure. 

Figure 2. TPR Profile Performed at Atmospheric Pressure

Figure 3. TPR Profile Performed at 25 bar Pressure

Figure 3 showing the 25-bar pressure TPR profile features a sharp inverted peak at 150°C and a wider inverted peak close to 250°C. During reduction H₂ is consumed and mixes with the oxygen atoms in the metal oxide after the chemical reactions. 

As expected, the high pressure of H₂ over the catalyst considerably reduces the reduction temperature. The increased pressure provides a greater driving force for interaction between oxide and H₂ especially with respect to porous catalyst supports. 

Given that TPR is a bulk reaction, quantitative analysis can be performed with the mass spectrometer data to ascertain the overall quantity of reducible species existing in the catalyst. 

Experiment 2: Pulse Chemisorption (PC)

PC is an analytical method used to determine the accessible active species present on the catalyst surface. At room temperature H₂ was pulsed onto a catalyst consisting of approximately 800mg of 0.5 wt% Pt/Al₂O₃. The quantity of active gas adsorbed was then measured. In this study active gas was pulsed by means of a six-way valve with a 0.5mL STP loop in a gap of 3 minutes. To ensure fast and complete filling the 10% hydrogen balance nitrogen mixture was fed via the loop at a 20 sccm flow rate. Nitrogen at atmospheric pressure was used as a carrier gas. 

The data acquired from the mass spectrometer during the time of the experiment is shown in Figure 4. The five peaks denote the five pulses. The last three peaks are similar and are used as a baseline to recalculate the quantity of gas adsorbed from the initial two pulses. Combining the peaks and integrating this data with the known pulsed volume enables determination of the total quantity of gas adsorbed. 

Figure 4. Pulse chemisorption profile of H₂ on 0.5 wt% Pt/ Al₂O₃

The 0.5 wt% Pt/ Al₂O₃ catalyst is a standard utilized for Micromeritics Chemisorption equipment such as the AutoChem II 2920 and has a dispersion of 35% ± 5%. The percent dispersion measured from the data obtained through the mass spectrometer and MCCTC produced 36%. In cases where the PC experiment is carried out at increased pressures more amounts of active species will be adsorbed and hence the percent dispersion of the catalyst will be relatively higher. 

In addition, the PC technique can be used to determine other parameters such as the size of the active particles and active metal surface area. As the active particle size increases the catalyst’s activity decreases. 

Catalyst Characterization Experiments Using MCCTC

Temperature-Programmed Desorption (TPD)

TPD is a technique that monitors the quantity of gas desorbed from a catalyst surface during a linear temperature ramp following surface saturation by an active gas. To describe this analysis a TPD profile of H₂ on platinum was obtained on an Autochem II 2920 (Figure 5). 

The Autochem II 2920 utilizes a thermal conductivity detector (TCD) to measure the quantity of hydrogen in the exhaust stream. The same TPD profile is obtained through the MCCTC coupled with a mass spectrometer. The increasing signal denotes the release of hydrogen from the catalyst surface. The various peaks denote different active sites on the catalyst while peaks appearing at higher temperatures reflect sites having higher adsorption enthalpies. 

Figure 5. TPD Profile of H₂ on a platinum supported alumina catalyst

Temperature-Programmed Oxidation (TPO)

TPO is utilized to determine the extent of reduction of active species existing on the catalyst. Combining TPO and TPR techniques helps in assessing the number of regeneration cycles that a catalyst can experience before becoming fully deactivated. 

Figure 6. TPO profile of V₂O₅, a de-NO_x catalyst

TPO can also be employed for characterizing TiO₂, V₂O₅, and other de-NOx catalysts. Such catalysts are utilized for transforming toxic NOx contaminants into non-polluting components. Figure 6 illustrates an example of a standard TPO profile of a pre-reduced V₂O₅ catalyst. 

Conclusion

The MCCTC is a novel research tool that integrates almost every catalyst characterization method with catalytic activity testing. Performing these many tasks using a single instrument in place of multiple instruments enables significant cost savings. A valuable additional feature is the ability to distinguish catalysts at different temperatures and pressures, thus allowing the catalyst to undergo preliminary characterization and testing using the same reaction conditions. 

This not only predicts catalyst behavior prior to activity testing but also determines the cause of catalyst deactivation by means of post-activity testing. Users can thereby save both time and money by using a single system to characterize and test catalytic activity. 

This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.

For more information on this source, please visit Micromeritics Instrument Corporation.